Hackers acquired more than 600 gigabytes of data, including secret plans to develop a supersonic anti-ship missile to use on U.S. submarines by 2020, the Washington Post first reported.

The data concerned a project called Sea Dragon, signal information, submarine radio room information about cryptographic systems and a naval electronic warfare library.

Officials speaking on condition of anonymity told the Post the breach took place in January and February targeted an unidentified contractor with the Naval Undersea Warfare Center, based in Newport, R.I. The center researches and develops submarines and other underwater weaponry. An investigation is ongoing.

The information was in the contractor’s unclassified network, but officials said that the data could be considered unclassified when aggregated.

Lt. Marycate Walsh, a Navy spokeswoman, wouldn’t provide details to the New York Times beyond citing “measures in place that require companies to notify the government when a cyberincident has occurred that has actual or potential adverse effects on their networks that contain controlled unclassified information.”

The hack disclosure comes a week after U.S. Defense Secretary Jim Mattis criticized Beijing’s militarization of disputed islands in the South China Sea at a regional security conference.